1. Field of the Invention
The present invention relates to an image reading apparatus such as a scanner or optical character recognition apparatus, and more particularly, to a structure for correcting an output signal from a CCD image sensor in accordance with a reference level.
2. Description of the Related Art
Conventionally, there is a character recognition apparatus (hereinafter referred to as OCR) in which a sensor unit of a reduced-imaging type is located on a transportation path along which sheets are transported. The OCR of this type recognizes image information, such as characters and figures, recorded on the sheets by optically reading the data by means of the sensor unit.
In the sensor unit of the reduced-imaging type, light from a light source is applied to each sheet when a reading position on the transportation path is reached by the sheet, and reflected light from the sheet is guided to a CCD image sensor through an optical system that includes reflection mirrors, lenses, etc. Thus, the image information is delivered as shading information to the image sensor, whereupon it is converted into an electrical output signal.
In the OCR constructed in this manner, the level of the output signal of the CCD image sensor is corrected during the reading of image data so that the image information can he read accurately. This correction includes shading correction, gain adjustment, and light modulation. The correction is carried out by means of a white reference plate for use as a reference for color balance control. In the shading correction, more specifically, the output signal of the CCD image sensor is electrically corrected so that output voltages from all photoelectric conversion elements of the image sensor are on the same level when reflected light from the white reference plate is applied to the image sensor. In the gain adjustment, the amplification degree of a signal amplifier is adjusted so that the level of the output signal of the CCD image sensor after the shading correction is fixed. According to this gain adjustment, image information on a white-ground sheet can be correctly read without regard to its reflectance. Therefore, this adjustment is essential correction for color image information to be read. The gain adjustment may be replaced with the technique of adjusting the quantity of light applied from the light source to the reading position to correct the level of the output signal of the CCD image sensor. Both the gain adjustment and the light-quantity adjustment may be performed.
In the conventional OCR, the quantity of light from the light source may change over time with use, or the level of the output signal of the CCD image sensor may change owing to deterioration of the image sensor or temperature of the work environment. Accordingly, the CCD image sensor cannot deliver a constant output signal every time the image data is read.
Conventionally, therefore, an OCR is known such that the white reference plate is located in the reading position of the transportation path. In this OCR, light from the light source is applied to a white face of the white reference plate at the start of its operation, and the quantity of light reflected by the white face and the RGB color balance are measured by means of the CCD image sensor. Correction control of read data, such as the shading correction, is executed in accordance with the output signal of the CCD image sensor.
According to an OCR described in Jpn. Pat. Appln. KOKOKU Publication No. 7-113952, the white reference plate is movable between a first position in which it is advanced to the reading position and a second position off the reading position. When the white reference plate is moved to the first position, its white face is kept at a standstill in a reading line position of the CCD image sensor, and the reference level is fetched. As the image information is read from a sheet, the white reference plate is moved from the first position to the second position and kept off the reading line position of the image sensor.
This OCR is furnished with a drive mechanism for moving the white reference plate alternatively to the first or second position. The drive mechanism includes a solenoid for use as a drive source, a plurality of links for coupling the solenoid and the white reference plate, and a return spring for urging the white reference plate toward the second position. The solenoid is turned on when the light source glows so that the correction control of read data can be executed. If the solenoid is turned on, the links convert its linear reciprocation into a rocking motion, which is transmitted to the white reference plate. Thereupon, the white reference plate moves from the second position to the first position, resisting the urging force of the return spring, and is held in the first position. When the fetch of the reference level is finished, the solenoid is turned off. Consequently, the white reference plate is forced to return to the second position by means of the return spring.
According to the conventional drive mechanism, however, the solenoid must be turned on and off every time the white reference plate is rocked from the first position to the second position or in the opposite direction. Therefore, the motion of the white reference plate is intermittent, so that rocking it from the first position to the second position takes several seconds. Since the white reference plate must be kept stationary in the first or second position, moreover, it takes time to rest the reference plate having started to rock. Thus, the white reference plate cannot be continuously rocked at high speed.
Since the linear reciprocation of the solenoid is converted into the rocking motion by means of the links in the conventional drive mechanism, moreover, the white reference plate may possibly be caused to shake by clearances that are created at the joints of the links. Accordingly, it is hard to hold the white reference plate exactly in the first position.
In transporting a plurality of sheets one after another at high speed, therefore, the conventional drive mechanism cannot rock the white reference plate corresponding to the speed of sheet transportation. Conventionally, in consequence, the correction control of the read data can be executed only at the start of operation of the OCR. If the level of the output signal of the CCD image sensor varies during the sheet reading operation, therefore, the output signal cannot be corrected. Thus, the reading accuracy may lower, or the read image information may be unstable.
On the other hand, a white reference plate that is described in Jpn. Pat. Appln. KOKAI Publication No. 4-208935 or 6-291938, for example, moves to the first position before reading the image information starts. This white reference plate moves to the second position and retreats from the transportation path before a sheet reaches the reading position on the transportation path. At the point of time when the image information starts to be read, therefore, the sheet and the white face of the white reference plate are kept out of contact with each other, so that the reference plate can be prevented from being soiled.
According to the prior art described above, the white reference plate is held in the second position off the transportation path while the image information is being read. However, the second position in which the white reference plate is held is set under the transportation path. Even when the white reference plate is moved to the second position, therefore, it is opposed to the sheet that is transported along the transportation path.
If any foreign matter, such as paper dust, scraps of paper, or dust adhering to the sheet, is separated from the sheet during transportation, therefore, it may drop onto the white reference plate, in some cases. As the number of sheets to be read increases, the foreign matter more easily adheres to and remains on the white reference plate. The white reference plate described in Jpn. Pat. Appln. KOKAI Publication No. 6-291938 maintains its substantially horizontal posture as it advances into or retreats from the sheet transportation path. In this case, in particular, the white face of the white reference plate is always opposed to a sheet. Once any foreign matter adheres to the white face of the white reference plate, therefore, it cannot be white face separated from the reference plate, so that the white face or the reference plate is inevitably soiled soon.
In consequence, accurate correction control of color balance based on white color cannot be executed with ease, so that reading errors may occur, or the read image information may be unstable.
Restraining the occurrence of reading errors requires frequent cleaning of the white reference plate. Accordingly, the maintenance of the image reading apparatus takes much time and labor, so that a user's burden increases inevitably.